Prolog never delivered on the promise of declarative programming, because its rigid, unintelligent depth-first search strategy makes it necessary to understand the execution model and keep it in mind when writing programs. So it's only mostly declarative. Prolog achieved some popularity because there are times when brute-force depth-first search is all you need, particularly if it can be done very fast; but once you bump against the limitations of that paradigm, Prolog is of little help.

True declarative programming is still an open research problem.